Is it a……

humidity issue?  A change in the humidity does make wood warp.  You mentioned you have the workbench in a garage.  It might tend to be more humid there.  It would be interesting to see what happens to it if it returns to your Father-in-law’s work area to re-establish the recent humidity level.

The dreaded warpage

I had a free standing bit of model railroad benchwork twist so one corner leg was a couple inches in the air once.  I used some spare lumber up against the rafters to wedge the two opposite corners down, and after a while it seemed to get stable and I could remove them.  No idea if I just got lucky or if that would work for you too.

Stumpy Nubs has a couple good videos on wood warpage which changed the way I sort through the pile at home depot:

Construction

First, I'll caveat my comments with the fact that I consider myself a beginner in woodworking, and I might mess up some terms here, but...

If the bench top was constructed with solid wood instead of plywood and without considering how that solid wood expands and contracts in humidity / temperature, this could be the issue.

Likely the grain of the wood is in line with the long side of your table. i.e. the grain lines run from left to right or are 'stacked' front to back in the picture. These are like small tubes that absorb moisture (the tree used them to pull moisture from roots to the leaves). If they swell from humidity or temperature they tend to swell in the 'front to back' direction in your photo (the garden hose fills up with water and gets wider). Meaning it will get wider and thinner front to back as moisture in the wood increases or decreases. If you screwed the table top into the apron all the way around, when the table swells, it will push out against the apron and bust your table. For fastening table tops like this to the apron or base of the table, you should fasten in only a few places near the middle on either end with a screw, or something that doesn't give, the rest of your fasteners should be able to move with the expansion / contraction of the wood (like a z clip in a dado or other methods).

Also, if the work bench wasn't sealed to prevent increases / decreases in humidity, it needs to be sealed on all sides with tongue oil, polyurethane, epoxy, etc... many options there depending on your use of the table and the material you build it with.

Building with plywood will reduce your expansion / contraction issues. Plywood is made of many thin layers of wood that are stacked in layers perpendicular to each other. The grain pattern alternates with each thin sheet. This makes the material more stable as it resists expansion / contraction in both directions. The glue between the layers helps even more. The more layers in your plywood, the better it will fair, but it will cost more (generally).

A high grade plywood that is sanded on at least one side, stacked in several layers might do the trick for you as a basic workbench. You can also use MDF sheets (nice and flat) but you have to protect it from moisture and band the edges with a hardwood or plywood. If you're going to work with MDF, or any wood for that matter, do it outside and wear a high quality respirator (and shave your beard), if you don't have a really good dust collection system. Hobbyist wood workers often don't realize the long term damage they are doing to their lungs. Even a simple project in a garage will expose you to more dust hazard and do more damage to your lungs than a professional shop worker receives in months on the job in a regulated facility. 

Router sled

Well the first problem that I see is the assumption that HD sell dry wood.  It don't period.  The wood they get is sold as fast as it get into the system.

Second the bench was not sealed as soon as it was finished.

Third you moved it from one location to another.

So now what

first wait a while, 2 months to a year for the wood to dry out and stop warping.

then when you think it is as dry as it going to use a router sled to flatten the top.

Then make sure you seal the top.

Btw old (100y) old work benches have to be flatten every now and again in their lifetime.

Dan

Grain

It can also be that all the grain in the boards is running the same way.  Boards will naturally "cup" and if you glue them all together with the "cupping" running the same way it will get worse.  If you alternate the cupping direction in adjacent boards, it will minimize the effect.  You might be able to remove the top, rip it into two or three pieces  and then reverse every other one to get it closer to flat.

Thank you all for the

Thank you all for the informative responses. My father-in-law is a great guy. He's a builder but not really a woodworker. The top of the work bench is 2x6 ,2x8, and 2x12’s cut to fit and bolted to a frame. I think I'll remove the boards and replace with a plywood/ MDF sandwich which I will glue and screw together. I think a 2.5” to 3” thickness will work fine.  I’ll use hard wood 1x3’s on the edges.  I think I’ll add several full length T Track sections as well.

I drove by Lowe's on the way home from work today and priced the plywood and MDF and all I have to say is WOW!  I’ll check several of the other suppliers in town as well. 

Solid top

  Can't tell from the photos but how warped is it and how was the top installed?  I'd try to keep the top if possible , a 2X6- 2X8,etc. top is plenty strong for model railroad work. If each board is reasonably straight you may just need to remove them and re-space them a bit so they have some expansion room. Heavy tops like this don't need to be glued into one big slab, they can just be minimally fastened from below so the individual boards have room to adjust to seasonal changes. You don't really need a totally smooth large top for most model building, you might just be able to get a large flat cutting board and lay it on top when you need a perfectly smooth work area.....DaveB  

Hi, this is a very nice

Hi,

this is a very nice workbench!

In some Videos I've seen on workbench construction, they used 2x6 boards but joined them standing up, so that the top would be (almost) 6 inches of solid wood. But that is not the kind of workbench you're after...

In your case, I would get a piece of (heavy) plywood and a couple of shims. Just use the shims to make the plywood lie flat on your current top and then screw it tight from below. The extra weight is likely to benefit your bench in terms of stability. You'll likely be able to drive a nail in the middle of the bench if it is sufficiently stable, otherwise anything you try with a hammer will just result in bouncing...

Of course, a full replacement is also an option. You could also plane the bench flat and then attach a plywood topper.

Have fun.

What are you going to be using it for?

being on a model railroad forum I'm going to assume light to moderately heavy projects...nothing massive.......the framework under the top in question bears out that assumption.

I'd tear the top out, add some extra top framing, cut a couple of 3/4" plywood pieces and laminate them together for a new 1.5" plywood top, glue and screw the plywood down, and then use the durn thing!

MDF isn't going to hold up as

MDF isn't going to hold up as a work surface if you going use it in a garage . MDF is just particle board , while fine for desk top or hobby work surface , if you start to beat on it with car parts ,hammers ,hoes,rakes , lawnmower bits ....ect , its going fail , and if you get it wet with so much as a bottom of a coffee cup it will swell and warts , if gets left outside in the rain , it disintegrate. 

I have used MDF for work surfaces inside , but first I coated it with 3M polyester resin , and then top coated with gel coat. I used 2-3 coats of polyester and 1-2 coats of gel , even then I had one or two pin holes , that when I when wiped it with a wet rag to clean , it swells up like a big butt pimple. 

The best way to construct a workbench is to frame it like you've done and then sheath it on 3 sides with 3/4 plywood if you don't it will rack/twist on you . It looks to me like your problem is lack of braces I only see 2 , you need X braces on 3 sides if your not going to use plywood  , the top can be 2x4 or 1 or 2 layers of 3/4 plywood ,with the edges  framed in 2x4.

If you use 2X4s for the top , they should be well acclimated ,edge planed ,and either biscuited or doweled or screwed together otherwise they warp independently of one another.

If your really going to beat on it torch it or weld on it, make it out of steel.

Might try this

Replace the top with 2 layers

Replace the top with 2 layers of 3/4 inch plywood. Glue and screw them together. Edge band the top with 1x material and leave it 1/4 inch higher than the top. This will give you a raised curb all around the top. Place a 1/4 piece of Masonite in the space that is left. Attach it with some screws and you will have an easily replaced work surface on a strong bench. Use tite bond 3 glue as it is water proof or a poly glue. Paint all surfaces of the plywood with a good exterior paint and all of the wood pieces to the bench. That will help eliminate or reduce the seasonal changes and preserve the bench. The higher quality plywood will have more plys and the voids will be filled. Resist the urge to go cheap. Quality is remembered long after price is forgotten.

Can you just unbolt it …

See how bad it’s sprung (including the frame), re-square the frame if it’s out, and shim the top flat, and then rebolt.  

Door / window shims are cheap and, after all, your pop-in-law made it for you, so its a gift right?  Every time you look at that top, you’ll think of him, versus some plain old plywood.

Even dry wood can have

Even dry wood can have interior tension and when cut the two parts are no longer fighting each other and the twist or now or warp or whatever.  This is the same thing that causes a board to pinch a saw blade.  
Other the replacing it or flattening with a router or hand plane not much you can do

-Doug M

Lowe's has Butcher Block Tops

I noticed the other day that Lowe's has pre-made, plastic wrapped Butcher Block tops in two different lengths for sale. I didn't check the price.

"The top warped the frame and

IMHO

I think it depends on what this workbench is for. The 3, 4 and six-inch solid wood tops are, at least to me, associated with woodworking, not model railroading. Perhaps you intend to do woodworking on it too, but if not, I think a double layer of plywood should be sufficient. I liked Rob's idea of two layers of plywood topped with masonite - that would let you use plywood that's got some knots instead of having to spring for cabinet grade stuff.

I'm not sure I'd want screws exposed on the top of the work surface though, even countersunk. I'd probably just glue the top layer down and clamp around the edges and weight the center. If it gets too torn up, you can always put another layer of masonite on top later without removing the old layer.

Sacrificial bench top - carpet tape

I’ve got a 3x6 foot bench I wanted a sacrificial top layer on, so I could feel OK about doing track saw cuts right on it, drilling into it a little, etc.  I got 3 2x4 foot 1/4” thick MDF panels, and used carpet tape to stick them to the bench top, then trimmed off the overhang.  Just a couple strips of 2” tape per panel.  The carpet tape holds them very securely, and there are no fasteners of any kind to worry about hitting with the saw or whatever.  But when it gets too beat up I’ll be able to pry the old MDF pieces off and replace them with new ones.

sacrificial bench top

   I have a work bench in my wood shop that I built over 40 years ago .The top is made with 2 by 4 studs set on edge and nailed together. To smooth the top a bit I put a sacrificial piece of 1/4 inch masonite on top but have never needed to change it. It has a few extra holes where I've drilled for temporary bench stops or clamps and a few gouges and saw cuts but still quite usable as is. I just fastened the masonite with a few brad nails around the edge ...DaveB

Thank you for all the great

Thank you for all the great suggestions!  The intended use is a roll around workbench with storage for several bench top power tools.  I would like to clamp wood to it for cutting, routing as well.  I have another use for the boards that currently make up the top so I’ll replace with a plywood top. I have several sections of T-Track I’d like to install as well. I’m not sure how that would work with the replaceable thin top idea. Which I like!